1. Field of the Invention
The present invention relates generally to a gas-insulated electric apparatus, e.g. a gas-insulated transformer using a high withstand voltage insulating gas such as SF.sub.6, and more particularly to a gas-insulated electric apparatus having a radiator for cooling the high withstand voltage insulating gas.
2. Description of the Related Art
Recently, a transformer station is often constructed within an office building or in a basement. In the transformer station, a high voltage electric apparatus such as a power transformer is installed. In a conventional high voltage electric apparatus, an insulating oil has been used as a cooling medium. The insulating oil has a problem of safety, e.g. fire. Under the situation, in these years, SF.sub.6 gas has been used in a high voltage electric apparatus. The SF.sub.6 gas has been used not only as a high withstand voltage insulating gas but also as a cooling medium. Such a gas-insulated electric apparatus comprises an electric apparatus body and a radiator attached to the body.
As is well known, however, the specific heat and conduction of SF.sub.6 gas is lower than that of the insulating oil. Since the heat transmission performance is much inferior to that of the insulating oil, it is necessary to use a large-capacity radiator. In addition, the space in the office building or basement, where the high voltage electric apparatus is to be installed, is limited; thus, it is difficult to install the high voltage electric apparatus with a large radiator.
FIG. 1 shows an example of a conventional self-cooling type gas-insulated transformer, which is a typical example of the above gas-insulated electric apparatus. In FIG. 1, the self-cooling type gas-insulated transformer 10 comprises a transformer body 12 and a radiator 14. Main components of the transformer body 12 are a casing 12A, a coil 12B and an iron core 12C. The coil 12B and iron core 12C are situated within the casing 12A in the insulated state. A high withstand voltage insulating gas or SF.sub.6 16 is filled in the transformer body 12 and radiator 14.
The radiator 14 will now be described in detail with reference to FIGS. 2 and 3. A plurality of mutually distanced panels 14B, each having substantially the same thickness, are coupled between an upper header 14A and a lower header 14A, which have an oval cross section, via couplers 14C. The couplers 14C are provided at both end portions of each panel 14B. The couplers 14C, on the other hand, are attached to the mutually facing surfaces of the upper and lower headers 14A. The couplers 14C control branching and confluence of insulating gas 16 which flows through the panels 14B.
An open end portion of each of the upper and lower headers 14A is provided with a flange 14D1, 14D2. The flanges 14D1 and 14D2 are connected to a transformer body (not shown). Thereby, the inside space of the transformer body communicates with the inside space of the upper header 14A and the inside space of the lower header 14A. The other end portions of the upper and lower headers 14A are closed Each panel 14B has a longitudinally extending inside space. The inside space of each panel 14B communicates with the inside space of the upper header 14A and the inside space of the lower header 14A. Accordingly, a closed gas passageway is formed by the mutually communicating inside spaces of the transformer body, upper and lower headers 14A and panels 14B.
The SF.sub.6 gas 16 filled in the closed gas passageway circulates naturally through the closed passageway, and radiates heat in the panels 14B principally, thereby cooling the inside spaces of the transformer body and radiator 14. The natural circulation of SF.sub.6 gas 16 will now be described more specifically. The SF.sub.6 gas 16 flows to a passageway 18A1 of the upper header 14A from the transformer body. Then, the gas 16 is branched into the panels 14B, flowing vertically downwards through passageways 18B of the panels 14B. The SF.sub.6 gas 16 flowing through the passageways 18B of panels 14B is made confluent in a passageway 18A2 of the lower header 14A. The confluent SF.sub.6 gas 16 returns to the transformer body.
In the above, when the SF.sub.6 gas flows through the passageways 18B of the panels 14B, the air around the panels 14B is heated and convection occurs By the convection, heat radiation is principally caused. When the gas 16 flows in the passageways 18B of panels 14 in turbulent flows, radiation efficiency is increased.
In this case, since SF.sub.6 gas having less heat transfer performance is substituted for the insulating coil as a cooling medium, it is necessary to increase the circulation amount of SF.sub.6 gas, thereby to enhance the cooling performance of the radiator 14.
If the ratio of the cross section area of the passageway 18A1, 18A2 of each of the upper and lower headers 14A to the cross section area of each coupler 14C between each panel 14B and upper and lower headers 14A, at which SF.sub.6 gas is branched or made confluent, is large, the branch loss coefficient and confluence loss coefficient are high. In this case, the following disadvantage arises, and the size of the radiator 14 cannot be reduced.
First, the branch loss/confluence loss at the coupler 14C is expressed by the product of the square of the flow velocity of SF.sub.6 gas at the passageways 18A1 and 18A2, the density of SF.sub.6 gas and the branch loss coefficient or confluence loss coefficient; thus, if the branch loss coefficient or confluence loss coefficient increases, the branch loss or confluence loss increases or the circulation flow rate of SF.sub.6 gas decreases.
Secondly, if the branch loss or confluence loss increases, the flow rates of SF.sub.6 flowing through the panels 14B tend to become non-uniform, and a laminar flow of SF.sub.6 gas with low heat conductivity may occur in some of the panels 14B. In such a case, even if the number of panels 14B is increased, the radiation amount does not substantially increase.
A second example of prior art will now be described with reference to FIGS. 4 and 5. As is shown in FIGS. 4 and 5, a radiator 20 is connected to a transformer body of a self-cooling type gas-insulated transformer (not shown). The radiator 20 has a pipe-like upper header 20A, a pipe-like lower header 20A, and a plurality of mutually distanced panels 20B situated between the upper and lower headers 20A. Each panel 20B has substantially the same thickness. Each of the upper and lower headers 20A has a plurality of ducts 20C along its longitudinal direction. A hole is formed at both end portions of each panel 20B. The upper and lower headers 20A are inserted through the holes formed at both end portions of the panels 20B. The positions of the holes at both end portions of the panels 20B are made to agree with the positions of the ducts 20C of the upper and lower headers 20A, and the panels 20B are coupled to the upper and lower headers 20A by means of welding, etc. The ducts 20C of the upper header 20A are opposed to the ducts 20C of the lower header 20A. The ducts 20C of the upper and lower headers 20A control the branching and confluence of the insulating gas 16 flowing through the panels 20B.
An open end portion of each of the upper and lower headers 20A is provided with a flange 20D1, 20D2. The flanges 20D1 and 20D2 are connected to the transformer body (not shown). Thereby, the inside space of the transformer body communicates with the inside spaces of the upper and lower headers 20A. The other end portion of each of the upper and lower headers 20A is closed. Each panel 20B has a longitudinally extending inside space. The inside spaces of the panels 20B communicate with the inside spaces of the upper and lower headers 20A. Accordingly, a closed gas passageway is formed by the mutually communicating inside spaces of the transformer body, upper and lower headers 20A and panels 20B.
The SF.sub.6 gas filled in the closed gas passageway circulates naturally through the closed passageway, and radiates heat in the panels 20B principally, thereby cooling the inside spaces of the transformer body and radiator 20. The natural circulation of SF.sub.6 gas will now be described more specifically. The SF.sub.6 gas flows to a passageway 22A1 of the upper header 20A from the transformer body. Then, the gas is branched into the panels 20B, flowing vertically downwards through passageways 22B of the panels 20B. The SF.sub.6 gas flowing through the passageways 22B of panels 20B is made confluent in a passageway 22A2 of the lower header 20A. The confluent SF.sub.6 gas returns to the transformer body.
In the above, when the SF.sub.6 gas flows through the passageways 22B of the panels 20B, the air around the panels 20B is heated and convection occurs. By the convection, heat radiation is principally caused. When the gas flows in the passageways 22B of panels 20B in turbulent flows, radiation efficiency is increased.
In this case, since SF.sub.6 gas having less heat transmission performance is substituted for the insulating coil as a cooling medium, it is necessary to increase the circulation amount of SF.sub.6 gas, thereby to enhance the cooling performance of the radiator 20.
It was thought that, in order to smooth convection of air around the panels 20B and enhance the heat exchange performance of the panels 20B, the outside diameter of each of the upper and lower headers 20A, which obstruct convection, is reduced. However, if the outside diameter of each of the upper and lower headers 20A is decreased, the inside diameter thereof is also decreased and the cross section area of the passageway 22A of each header 20A is decreased. Thus, it is disadvantageous, as in the first example, to decrease the outside diameter of each of the upper and lower headers 20A, and it is difficult to decrease the size of the radiator 20.
On the other hand, the self-cooling type gas-insulated transformer of the second example, which uses the cooling medium such as insulating oil or insulating gas, is widely employed in medium- and small-capacity transformers. In the case of the self-cooling type transformer, however, the circulation force of the cooling medium for cooling the coil and iron core is weaker than that of a forced-circulation type apparatus; thus, it is necessary to reduce the pressure loss as low as possible, increase the circulation amount of cooling medium as much as possible, and let the cooling medium flow through the passageway for cooling the coil and iron core with a highest possible efficiency. If the circulation amount of cooling medium is small and the circulation efficiency of cooling medium caused to flow through the passageway for cooling the coil and iron core is low, the size, cost and installation space of the transformer must be increased.
As is shown in FIG. 1, the SF.sub.6 within the transformer body flows, as indicated by broken-line arrows, through not only the passageway provided in the coil 12B and iron core 12C but also the space between the coil 12B and casing 12A, thereby to cool the coil 12B and iron core 12C. The flow of SF.sub.6 gas 16 through the space between the coil 12B and casing 12A, however, does little contribute to cooling the coil 12B.
Next, a problem arising when SF.sub.6 gas 16 flows through the space between the coil 12B and 12A will now be described. Suppose that the flow rate of the SF.sub.6 gas flowing through the passageway for cooling the coil 12B and iron core 12C is W1, and the flow rate of the SF.sub.6 gas flowing through the space between the coil 12B and casing 12A is W2. In this case, SF.sub.6 gas 16 of W1 and W2 flows in the radiator 14. In order to prevent an increase in pressure loss in the radiator 14, it is necessary to increase the size of the radiator 14, which will be situated in a larger installation space, thereby preventing a decrease in circulation flow amount. Further, in order to increase W1, it is necessary to increase the cross section area of the passageway for cooling the coil 12B and iron core 12C.
As stated above, in the conventional transformer 10, the SF.sub.6 16 flows through the space between the coil 12B and casing 12A; consequently, the installation space for installing the radiator 14 and the space between the coil 12B and iron core 12C increase, resulting in an increase in size and cost of the transformer 10.